Enriching Silicon-28 to >99.9999% for Quantum Information

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Summary

Within the field of quantum computing research, silicon wafers have been identified as a natural substrate on which to form qubits using gate defined quantum dots or embedded dopant atoms. Natural silicon has several isotopes, but the silicon-28 isotope is the most abundant and most favorable due to its lack of a nuclear spin, which can cause decoherence of an electron or nuclear qubit formed on a silicon substrate. For this reason, highly enriched silicon-28 is a valuable material for semiconductor qubits and the enriched state directly accounts for substantial technical advancements. In this project, we have developed a process for growing silicon-28 epilayers on top of a natural abundance silicon substrates. The silicon-28 is isolated by first ionizing natural abundance silicon from silane gas and then passing the ions through a magnetic mass analyzer. The deposited epilayers are more than 99.9999% enriched silicon-28, the most highly enriched silicon in the world. However, further improvements and technical demonstrations are still needed to develop material suitable for quantum information devices.

Description

Figure 1. Secondary Ion Mass Spectroscopy (SIMS) data that measures the abundance of the three isotopes of silicon as a function of depth in the film.

We are depositing, assessing and fabricating test devices built from highly enriched silicon to obtain data for improving the material quality and, ultimately, enabling the fabrication of devices that can exhibit quantum coherence. The highly enriched nature of the material is established via ex situ Secondary Ion Mass Spectrometry (SIMS) measurements, an example is shown in Figure 1 for a crystalline film of enriched silicon. These results show the material has less than 1 ppm of 29Si.

Figure 2. Scanning tunneling microscope image of a 28-Si film deposited onto a Si(100) substrate. The crystalline features of the Si(100), such as the 2x1 reconstruction, are visible in the film.

For use in quantum information, the high enrichment must also occur in an ultra-clean, crystalline form. We seek to deposit enriched silicon-28 as epilayers with a two-dimensional morphology, lattice matched to commercial silicon substrates with low defect densities. An STM image taken on the surface of a deposited silicon-28 film is shown in Figure 2. The sharp contrast differences are due to steps of a single atomic height and the 2x1 reconstruction typical of the Si(100) surface is faintly visible.

We are working to make devices from enriched substrates and films that are designed to optimize quantum behavior. As part of our material characterization, we are fabricating Schottky diodes, MOSCAP capacitors and Hall bars for measurements of carrier density, mobility, and defect densities. These classical electronic devices are precursors to quantum dot and single electron devices. One method we are developing to pattern nanometer scale features in situ uses hydrogen masks defined with voltage pulses from the STM. We are also pursuing the fabrication of quantum devices (and back-end fabrication) by electron beam lithography and conventional clean room processing.